See https://github.com/BLAKE2/libb2/issues/12.

All Blake2 params have to be encoded in little-endian byte order. For the two multi-byte integer params, leaf_length and node_offset, that means that assigning a native-endian integer to them appears to work on little-endian platforms, but gives the wrong result on big-endian. The current libb2 API doesn't make that very clear, and @sneves is working on new API functions in the GH issue above. In the meantime, we can work around the problem by explicitly assigning little-endian values to the parameter block.